The proposed studies exploit exciting new developments in neuroplasticity to enhance recovery of respiratory function following cervical spinal cord injury (SCI). There are about 12,000 new cases of SCI in the United States each year, with nearly 500,000 people affected. Most SCI's are incomplete with some sparing of spinal cord pathways. The number of injuries involving the cervical spinal cord is increasing, with resulting impairments of rhythmic phrenic nerve activity and paralysis of the diaphragm muscle. Inability to generate expulsive behaviors as well as long-term mechanical ventilation needed to support such SCI patients is associated with elevated infectious, morbidity and mortality rates. In recent studies supported by this grant, we found that the neurotrophin brain-derived neurotrophic factor (BDNF) acting through the full-length high affinity tropomyosin related kinase receptor subtype B (TrkB.FL) plays an important role in neuroplasticity and recovery of phrenic activity following SCI. This proposal will exploit a multi-pronged approach to gain mechanistic insight into the effects of BDNF/TrkB.FL signaling on: 1) strengthening of spared synaptic inputs to phrenic motoneurons, and 2) enhancing motoneuron survival post-SCI. Our working hypothesis is that increasing BDNF/TrkB.FL signaling in phrenic motoneurons enhances functional recovery of rhythmic phrenic activity after cervical SCI. Three complementary models of cervical SCI that differentially impact respiratory function will be used in combination with targeted intrapleural AAV-mediated delivery of TrkB.FL to phrenic motoneurons, localized transplantation of adult-derived mesenchymal stem cells engineered to release BDNF and a chemical-genetic approach using a TrkB knock-in mouse that permits rapid and selective inhibition of TrkB kinase activity in gain-of-function/loss-of-function study design. Phrenic motoneuron expression of excitatory glutamate and serotonin receptors correlates with the time course of functional recovery after SCI, and thus we hypothesize that BDNF/TrkB.FL acts on phrenic motoneurons to enhance recovery of rhythmic phrenic activity via expression of specific glutamate and serotonin receptor subtypes. Three specific aims are proposed: 1) To determine whether functional recovery of rhythmic phenic activity after SH is enhanced by targeting increased BDNF/TrkB.FL signaling to phrenic motoneurons; 2) To determine the role of glutamatergic and serotonergic neurotransmission in the BDNF/TrkB.FL mediated recovery of rhythmic phrenic activity after SH; and 3) To determine the role of enhanced phrenic motoneuron BDNF/TrkB.FL signaling on motoneuron survival and functional recovery following mid-cervical contusion injury. We will assess phrenic responses across a range of motor behaviors to maximize functional gains associated with recovery. Our long-term goal is to develop an effective, targeted therapy to increase BDNF/TrkB.FL signaling in phrenic motoneurons to strengthen spared synaptic inputs to phrenic motoneurons and promote motoneuron survival, thereby enhancing functional recovery following SCI.